The invention relates generally to the field of cytogenetics, and more particularly, to methods for identifying and classifying chromosomes.
Chromosome abnormalities are associated with genetic disorders, degenerative diseases, and exposure to agents known to cause degenerative diseases, particularly cancer, German, "Studying Human Chromosomes Today," American Scientist, Vol. 58, pgs. 182-201 (1970); Yunis, "The Chromosomal Basis of Human Neoplasia," Science, Vol. 221, pgs. 227-236 (1983); and German, "Clinical Implication of Chromosome Breakage," in Genetic Damage in Man Caused by Environmental Agents, Berg, Ed., pgs. 65-86 (Academic Press, New York, 1979). Chromosomal abnormalities can be of three general types: extra or missing individual chromosomes, extra or missing portions of a chromosome, or chromosomal rearrangements. The third category includes translocations (transfer of a piece from one chromosome onto another chromosome), and inversions (reversal in polarity of a chromosomal segment).
Detectable chromosomal abnormalities occur with a frequency of one in every 250 human births. Abnormalities that involve deletions or additions of chromosomal material alter the gene balance of an organism and generally lead to fetal death or to serious mental physical defects. Down's syndrome is caused by having three copies of chromosome 21 instead of the normal 2. This syndrome is an example of a condition caused by abnormal chromosome number, or aneuploidy. Chronic myelogeneous leukemia is associated with the exchange of chromosomal material between chromosome 9 and chromosome 22. The transfer of chromosomal material in this leukemia is an example of a translocation. Clearly, a sensitive method for detecting chromosomal abnormalities would be a highly useful tool for genetic screening.
Measures of chromosome breakage and other aberrations caused by ionizing radiation or chemical mutagens are widely used as quantitative indicators of genetic damage caused by such agents, Biochemical Indicators of Radiation Injury in Man (International Atomic Energy Agency, Vienna, 1971); and Berg, Ed. Genetic Damage in Man Caused by Environmental Agents (Academic Press, New York, 1979). A host of potentially carcinogenic and teratogenic chemicals are widely distributed in the environment because of industrial and agricultural activity. These chemicals include pesticides, and a range of industrial wastes and by-products, such as halogenated hydrocarbons, vinyl chloride, benzene, arsenic, and the like, Kraybill et al., Eds., Environmental Cancer (Hermisphere Publishing Corporation, New York, 1977). Sensitive measures of chromosomal breaks and other abnormalities could form the basis of improved dosimetric and risk assessment methodologies for evaluating the consequences of exposure to such occupational and environmental agents.
Current procedures for genetic screening and biological dosimetry involve the analysis of karyotypes. A karyotype is a collection of indices which characterize the state of an organism's chromosomal complement. It includes such things as total chromosome number, copy number of individual chromosome types (e.g., the number of copies of chromosome X), and chromosomal morphology, e.g., as measured by length, centromeric index, connectedness, or the like. Chromosomal abnormalities can be detected by examination of karyotypes. Karyotypes are determined by staining an organism's metaphase, or condensed, chromosomes. Metaphase chromosomes are used because, until recently, it has not been possible to visualize nonmetaphase, or interphase chromosomes due to their dispersed condition in the cell nucleus.
The metaphase chromosomes can be stained by a number of cytological techniques to reveal a longitudinal segmentation into entities generally referred to as bands. The banding pattern of each chromosome within an organism is unique, permitting unambiguous chromosome identification regardless of morphological similarity, Latt, "Optical Studies of Metaphase Chromosome Organization," Annual Review of Biophysics and Bioengineering, Vol. 5, pgs. 1-37 (1976). Adequate karyotyping for detecting some important chromosomal abnormalities, such as translocations and inversions requires banding analysis. Unfortunately, such analysis requires cell culturing and preparation of high Quality metaphase spreads, which is extremely difficult and time consuming, and almost impossible for tumor cells.
The sensitivity and resolving power of current methods of karyotyping, are limited by the lack of stains that can readily distinguish different chromosomes having highly similar staining characteristics because of similarities in such gross features as size, morphology, and/or DNA base composition.
In recent years rapid advances nave taken place in the study of chromosome structure and its relation to genetic content and DNA composition. In part, the progress has come in the form of improved methods of gene mapping based on the availability of large quantities of pure DNA and RNA fragments for probes produced by genetic engineering techniques, e.g., Kao, "Somatic Cell Genetics and Gene Mappings," International Review of Cytology, Vol. 85, pgs. 109-146 (1983), and D'Eustachio et al., "Somatic Cell Genetics in Gene Families," Science, Vol. 220, pgs. 9, 19-924 (1983). The probes for gene mapping comprise labeled fragments of single stranded or double stranded DNA or RNA which are hybridized to complementary sites on chromosomal DNA. The following references are representative of studies utilizing gene probes for mapping: Harper et al. "Localization of the Human Insulin Gene to the Distal End of the Short Arm of Chromosome 11," Proc. Natl. Acad. Sci., Vol. 78, pgs. 4458-4460; Kao et al., "Assignment of the Structural Gene Coding for Albumin to Chromosome 4," Human Genetics, Vol. 62, pgs. 337-341 (1982); Willard et al., "Isolation and Characterization of a Major Tandem Repeat Family from the Human X Chromosome," Nucleic Acids Research, Vol. 11, pgs. 2017-2033 (1983); and Falkow et al., U.S. Pat. No. 4,358,535, issued 9 Nov. 1982, entitled "Specific DNA Probes in Diagnostic Microbiology." The hybridization process involves unravelling, or melting, the double stranded nucleic acids by heating, or other means. This step in the hybridization process is sometimes referred to as denaturing the nucleic acid. When the mixture of probe and target nucleic acids cool, strands having complementary bases recombine, or anneal. When a probe anneals with a target nucleic acid, the probe's location on the target can be detected by a label carried by the probe. When the target nucleic acid remains in its natural biological setting, e.g., DNA in chromosomes or cell nuclei (albeit fixed or altered by preparative techniques) the hybridization process is referred to as in situ hybridization.
Use of hybridization probes has been limited to identifying the location of genes or known DNA sequences on chromosomes. To this end it has been crucially important to produce pure, or homogeneous, probes to minimize hybridizations at locations other than at the site of interest, Henderson, "Cytological Hybridization to Mammalian Chromosomes," International Review of Cytology, Vol. 76, pgs. 1-46 (1982).
Manuelidis et al., in "Chromosomal and Nuclear Distribution of the Hind III 1.9-KB Human DNA Repeat Segment," Chromosoma, Vol. 91, pgs. 28-38 (1984), disclose the construction of a single kind of DNA probe for detecting multiple loci on chromosomes corresponding to members of a family of repeated DNA sequences.
Wallace et al., in "The Use of Synthetic Oligonucleotides as Hybridization Probes. II. Hybridization of Oligonucleotides of Mixed Sequence to Rabbit Beta-Globin DNA, "Nucleic Acids Research, Vol. 9, pgs. 879-894 (1981), disclose the construction of synthetic Oligonucleotide probes having mixed base sequences for detecting a single locus corresponding to a structural gene. The mixture of base sequences was determined by considering all possible nucleotide sequences which could code for a selected sequence of amino acids in the protein to which the structural gene corresponded.
Olsen et al., in "Isolation of Unique Sequence Human X Chromosomal Deoxyribonucleic Acid," Biochemistry, Vol. 19, pgs. 241 9-2428 (1980), disclose a method for isolating labeled unique sequence human X chromosomal DNA by successive hybridizations: first, total genomic human DNA against itself so that a unique sequence DNA fraction can be isolated; second, the isolated unique sequence human DNA fraction against mouse DNA so that homologous mouse/human sequences are removed; and finally, the unique sequence human DNA not homologous to mouse against the total genomic DNA of a human/mouse hybrid whose only human chromosome is chromosome X, so that a fraction of unique sequence X chromosomal DNA is isolated.